1. Field of the Invention
This invention relates to a process for preparing unsaturated carboxylic acids from unsaturated aldehydes in the presence of a phosphorus-molybdenum-magnesium or calcium type catalyst.
2. Description of the Prior Art
Many processes have been suggested for producing an unsaturated carboxylic acid from an unsaturated aldehyde by a gas-phase oxidazation.
These include, for example, a process for producing acrylic acid from acrolein by using a catalyst consisting of Mo, V, W and silicon (Japanese patent publication No. 12129/1969) and a process for producing acrylic acid by using a catalyst consisting of P, Mo and As (Japanese patent publication No. 19260/1963).
Many processes for producing methacrylic acid have also been suggested. These include the processes of for example, U.S. Pat. No. 3,686,294 (a P-Mo-As catalyst), Japanese patent publication No. 10773/1973 (a catalyst containing Mo and Tl), U.S. Pat. No. 3,795,703 (a P, Mo and alkali metal series catalyst) and Belgian Pat. No. 817100 (a P, Mo and Sb series catalyst). However, from the viewpoint of industrial suitability, these processes are insufficient as regards the selectivity and life time of the catalysts employed.